The invention relates to a process for preparing polyphosphazene polymers containing fluoroalkoxy and/or aryloxy substituents. More particularly, the invention relates to an improved method for preparing substantially dry sodium salts of the alcohols employed in the preparation of such polyphosphazene polymers.
Polyphosphazene polymers containing various alkoxy and/or aryloxy groups are well known in the prior art as illustrated, for example, by U.S. Pat. Nos. 3,515,688; 3,700,629; 3,702,833; 3,838,073; 3,853,974; 3,856,712; 3,856,713; 3,888,800; 4,055,523 and 4,076,658. Such polymers have been conventionally prepared in the prior art by a two-step process which involves preparing the alkali metal salt of the desired aliphatic or aromatic alcohol (i.e., the alkali metal alcoholates) and then subsequently reacting the alkali metal salt with a linear polydichlorophosphazene polymer. The reaction scheme utilizing sodium as the alkali metal can be illustrated as follows: ##STR1## In the above reaction scheme, R may be a substituted or unsubstituted alkyl or aryl group and n is an integer of from 20 to 50,000 or more. Step 2 in the above reaction scheme is commonly referred to in the polyphosphazene art as a "derivatization" reaction.
The above process is advantageous in many respects and has permitted the preparation of polyphosphazene polymers containing a wide variety of substituted and unsubstituted alkoxy and/or aryloxy substituents as illustrated by the above-mentioned patents. However, the process nevertheless exhibits a number of serious disadvantages. Thus, as will be evident, the use of metallic sodium leads to difficult handling problems and requires rigorous exclusion of moisture from the reaction system. In addition, the use of metallic sodium is proscribed in cases where it is desired to employ polyfunctional alcohols in the reaction such as polyols, amino-alcohols, hydroxyacids, etc. due to uncontrollable side reactions. In fact, it has been found that the use of a halogenated aromatic alcohol such as p-chlorophenol in the reaction can result in the reduction of a portion of the p-chlorophenol to phenol thereby resulting in the production of an impure polymer. Moreover, since metallic sodium is hygroscopic in nature, the salt often contains small amounts of water despite efforts to exclude moisture from the system. As is well known in the polyphosphazene art, the presence of water in the reaction between the salt and the linear polydichlorophosphazene polymer should be avoided since its presence can lead to hydrolysis of a portion of the polydichlorophosphazene and even to possible gellation of the finished polyphosphazene polymer.
In addition to the aforementioned prior art relating to the use of alkali metals in the preparation of polyphosphazene polymers, certain prior art publications and patents disclose the use of alkali metal hydrides and alkali metal hydroxides including sodium hydroxide in the derivatization of cyclic chlorophosphazene oligomers.
Thus, the article entitled "Reaction of Phosphonitrilic Chloride With p-Nitrophenol" by Kober et al, Vol. 5, No. 12, Inorganic Chemistry (1966) describes the single stage reaction of the cyclic trimer (NPCl.sub.2).sub.3, p-nitrophenol and potassium hydroxide with water removal accomplished during the reaction. The article entitled "Fluoroalkyl Phosphonitrilates, A New Class of Potential Fire-Resistant Hydraulic Fluids and Lubricants" by Lederle et al, Journal of Chemical and Engineering Data, Vol. 11, No. 2, pages 221-228 describes the preparation of cyclic fluoroalkyl phosphonitrilates by a single stage reaction of the cyclic trimer (NPCl.sub.2).sub.3 or tetramer (NPCl.sub.2).sub.4 with a fluoroalcohol and sodium hydride or potassium hydroxide. The water resulting from the reaction was again removed during the reaction. The article entitled "Substituent Exchange and Carbon-Oxygen Bond Cleavage with Aryloxycyclophosphazenes" by H. R. Allcock et al, Journal of The American Chemical Society, 98:14 pages 4143-4149, July 7, 1976 discloses the ligand exchange reaction between aryloxycyclo phosphazenes and sodium p-nitrophenoxide prepared by reaction of p-nitrophenol and potassium hydroxide.
U.S. Pat. Nos. 3,453,235 and 3,459,838 to Klender as well as U.S. Pat. No. 4,117,041 to Guschl also describe the derivatization of cyclic chlorophosphazene oligomers by the reaction of such oligomers with alkali metal alcoholates prepared by reaction of alkali metals, alkali metal hydrides and alkali metal hydroxides. Thus, U.S. Pat. No. 3,453,235 to Klender at column 4, lines 21-32 discloses the use of metal hydroxides in the formation of the salts while U.S. Pat. No. 4,117,041 to Guschl in Example 1 shows the preparation of alkali metal alcoholates prepared by reacting various alcohols with potassium hydroxide.
In addition to the foregoing, U.S. Pat. No. 1,910,331 to Gesellschaft generally describes the preparation of alkali metal aliphatic monohydroxy alcoholates by reaction of an alcohol with an alkali metal hydroxide in which water formed by the reaction is removed by azeotropic distillation.
It should be noted that a portion of the alcohol utilized in forming the alcoholate in this patent is employed to effect azeotropic distillation. In contrast, the present invention utilizes a separate solvent or solvent mixture to form the azeotrope with water.
As will be evident from the above prior art, certain of the individual features of the process of the present invention are known in the prior art. However, the prior art described above does not appear to disclose a total process for preparing high molecular weight linear polyphosphazene polymers containing alkoxy and/or aryloxy substituents in which all of these features are combined.